BY RACHEL METZ AND JORDAN ROBERTSON, AP TECHNOLOGY WRITERS

SAN FRANCISCO -- Steve Jobs, the mind behind the iPhone, iPad and other devices that turned Apple Inc. into one of the world's most powerful companies, resigned as the company's CEO on Wednesday, saying he can no longer handle the job.

The move appears to be the result of an unspecified medical condition for which he took an indefinite leave from his post in January. Apple's chief operating officer, Tim Cook, has been named CEO.

In a letter addressed to Apple's board and the "Apple community," Jobs said he "always said if there ever came a day when I could no longer meet my duties and expectations as Apple's CEO, I would be the first to let you know. Unfortunately, that day has come."

Jobs' health has long been a concern for Apple investors who see him as an industry oracle who seems to know what consumers want long before they do. After his announcement, Apple stock quickly fell 5.4 percent in after-hours trading.

Earlier this month Apple became the most valuable company in America, briefly surpassing Exxon Mobil. At the market close Wednesday its market value was $349 billion, just behind Exxon Mobil's $358 billion.

The company said Jobs gave the board his resignation Wednesday and suggested Cook be named the company's new leader. Apple said Jobs was elected board chairman and Cook is becoming a member of its board.

Jobs' hits seemed to grow bigger as the years went on: After the colorful iMac computer and the now-ubiquitous iPod, the iPhone redefined the category of smart phones and the iPad all but created the market for tablet computers.

His own aura seemed part of the attraction. On stage at trade shows and company events in his uniform of jeans, sneakers and black mock-turtlenecks, he'd entrance audiences with new devices, new colors, new software features, building up to a grand finale he'd predictably preface by saying, "One more thing."

Jobs, 56, shepherded Apple from a two-man startup to Silicon Valley darling when the Apple II, the first computer for regular people to really catch on, sent IBM Corp. and others scrambling to get their own PCs to market.

After Apple suffered a slump in the mid-1980s, he was forced out of the company. He was CEO at Next, another computer company, and Pixar, the computer-animation company that produced "Toy Story" on his watch, during the 10 years before he returned.

Apple was foundering before he returned, having lost $900 million in 1996 as Microsoft Windows-based PCs dominated the computer market. The company's fortunes began to turn around with its first new product under his direction, the iMac, which launched in 1998 and sold about 2 million in its first 12 months.

Apple's popularity grew in the U.S. throughout the 2000s as the ever-sleeker line of iPods introduced many lifelong Windows users to their first Apple gadget. Apple created another sensation in 2007 with the iPhone, the stark-looking but powerful smart phone that quickly dominated the industry.

The iPad was introduced less than a year and a half ago but has already sold nearly 29 million units as it inspired myriad rivals in a tablet computer market that scarcely existed before Apple stepped in.

As Jobs was praised for his vision, concerns about his health persisted. The January leave was Jobs' third medical leave over several years. He had previously survived pancreatic cancer and received a liver transplant.

Shannon Cross, an analyst at Cross Research, said Cook is a good choice to replace Jobs.

"He has taken over for Jobs twice in two medical leaves and the company has functioned extremely well," she said, adding that Cook has been Jobs' "right hand guy" for many years.

Cross also said Jobs put in place a "culture of innovation" that will help Apple remain a creative force in the industry.

"Steve Jobs is an extremely strong leader and clearly has made Apple a leading consumer electronics company and one of the most innovative companies in the world," she said. "However, he didn't do it alone."

AP Technology Writer Barbara Ortutay in San Francisco contributed to this report.